Free-living chordates
The next most advanced animal in the evolutionary tree is the lancelet or amphioxus, which is more fish-like in appearance but also has a stiff rod or notochord. This animal is 50 mm in length has a well-developed segmented muscular system that allows it to bury itself quickly in the sand. This animal has no clearly defined head region, only a light-sensitive spot at the anterior, no heart only a few pulsating arteries, no fins or limbs but only a slight dilation at the hind end. The strong muscles rhythmically contract against the notochord and the animal is propelled forward in a series of waves. These lancelets and the larval tunicates therefore resembled each other and considerable argument arose as to which form was the most direct ancestor for the rest of the vertebrates. The embryonic development of many animals often reflects their phylogeny or ancestry. Consequently, larval termites resemble bristletails and larval horseshoe crabs resemble the segmentedtrilobites. It was therefore argued that the lancelet was the ancestor to the tunicates,
